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Vitamin Supplements

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Vitamins are small organic compounds that are required for normal metabolism and cellular function. They do not directly provide the body with energy, but instead participate in biochemical processes that serve many different purposes, including energy metabolism. The majority of vitamins cannot be produced by the body and are obtained through dietary sources. An average healthy adult with a balanced diet can achieve their daily vitamin requirements without additional supplementation.

Vitamin supplements are dietary supplements that contain (perhaps unsurprisingly) vitamins. They may consist of individual vitamins (e.g. vitamin D exclusively) or a combination of multiple compounds (e.g. multivitamins). They serve a role in the treatment of nutrient deficiencies in the context of illness and various health conditions and may be recommended by healthcare professionals for this purpose.

Vitamin supplements are also highly popular among healthy adults as supplements for general well-being [1]. They are often marketed for “the promotion of good health and disease prevention”, and are claimed to be useful for many other vague purposes, including:

  • Promoting "heart health"
  • "Strengthening" the immune system
  • "Supporting metabolism"
  • "Boosting" energy
  • Many others

In these cases, the supplements are typically taken irrespective of whether or not the consumer actually has a nutrient deficiency. Typically, such vague and poorly-defined claims are not very meaningful from a scientific point of view (e.g. what does “heart health” actually mean? How do they “support metabolism” beyond the function of vitamins obtained through your diet?).

Vitamin Claims

Many practitioners of natural/alternative medicine also promote the use of high-dose vitamin supplementation for the treatment and prevention of many health conditions that are not directly related to nutrient deficiencies. This includes serious illnesses such as cancer [2-4]. Unfortunately, there is no credible evidence to back up these claims.


Generally speaking, vitamin supplementation beyond dietary intake does not appear to work for the prevention of disease or the promotion of good health. For the average person with a healthy and balanced diet, vitamin supplementation is not necessary (and may even be harmful – see below).

However, in the case of clinical vitamin deficiencies and for specific populations/conditions (e.g. pregnant women, alcoholism, nutrient malabsorption, vegan diet, others), vitamin supplementation may be recommended by your doctor. Always talk to a healthcare professional before starting or stopping any vitamin supplementation or any other treatments.


Some clinical conditions associated with nutrient deficiency/increased nutrient needs (e.g. vitamin deficiency, pregnancy, alcoholism, nutrient malabsorption, others) may require vitamin supplementation. In those cases, a doctor will recommend the appropriate use of such supplements. The evidence provided below refers specifically to the use of routine vitamin supplementation for the promotion of general health and disease prevention in average healthy people.


Quick Definition: "Systematic Review"

A systematic review (and/or “meta-analysis”) is a scientific study that examines the agreement between many clinical trials. Instead of looking at a single study, systematic reviews attempt to analyse the totality of evidence related to a topic. By doing so, the true answer to a given question can be more accurately estimated. For example, a single study might indicate that a particular treatment is effective, leading readers to believe that this treatment works. However, what if three other studies conclude that the same treatment does not work? A systematic review might analyse the data from all four of these trials and conclude (correctly) that the totality of the evidence indicates that the treatment does not work (3 say no, only 1 says yes). The relative importance of each study is determined by things like study size (number of participants – more is better), chance for bias, consistency of measurements, and many others.

Because of their strength, systematic reviews are generally considered one of the highest standards of evidence. However, they are also not perfect, and are subject to the quality of the contributing clinical studies. Hence, if only low-quality studies are used, low-quality results will emerge (a.k.a. “garbage in, garbage out”).


Clinical Studies of Vitamin Supplementation

Systematic reviews of clinical studies demonstrate that there is no reliable evidence to suggest that taking vitamin supplements extends life, improves general health, or reduces the occurrence of disease (including cardiovascular disease and cancer) [1, 5-10]. When half of a large group of average people are given vitamin supplements and the other half are given sugar pills (placebos), there is typically no significant difference in their general health, risk of death, or disease development over time. In other words, there is no good reason for healthy people to take (or spend money on) vitamin supplements.

Additionally, regular supplementation with certain vitamins has actually been shown to cause possible harm [6, 11]. Clinical studies and systematic reviews have demonstrated that beta-carotene and vitamin E supplementation likely increase mortality (i.e. risk of death), and the same may be true of vitamin A [6, 11]. Medical practice guidelines therefore recommend against beta-carotene and vitamin E supplementation [8], and caution should be exercised when using vitamin A.


Multivitamins are highly popular health supplements, generating billions of dollars in sales every year for the supplement industry. They are widely promoted as being essential for good health and disease prevention. Many varieties of multivitamins exist, differing in their composition, target population/age group, price, and marketing claims.

Multiple systematic reviews, accounting for dozens of clinical trials and hundreds of thousands of patients, have concluded that there is no reliable evidence to support the use of multivitamins as supplements for general health or disease prevention [1, 5, 7]. Multivitamin supplementation does not appear to reduce the risk of cardiovascular disease or cancer, or change overall mortality [5, 7]. It also does not appear to affect cognitive function [9].

Therefore, for healthy adults with a balanced diet, there does not seem to be any reason to use (or purchase) multivitamin supplements [1].

Benefits of Multivitamins

Vitamin D

As mentioned above, supplementation with certain vitamins is recommended in specific situations. Vitamin D falls into this narrow category.

Vitamin D is important for normal bone health and the regulation of blood calcium levels. It is used in the treatment of osteoporosis and has been shown to reduce fractures and falls in elderly populations [12]. For this reason, public health bodies recommend that older adults (and other specific populations) consume between 800-1000 units (IU) of vitamin D daily, either through diet alone or with the help of supplementation [13-14]. Younger adults require less vitamin D (400-600 IU). For all groups, however, large doses of vitamin D are discouraged. It is suggested that long-term intake of greater than 4000 IU per day can have negative effects on health [13-14].

Some natural/alternative health practitioners may recommend high-dose (greater than 4000 IU per day) supplementation with vitamin D to promote general health and prevent disease. There is simply no reliable evidence to support this recommendation [12].

There is also no convincing evidence that vitamin D supplementation:

  • Prevents cancer [15]
  • Prevents respiratory infections [12]
  • Prevents or treats rheumatoid arthritis [12]
  • Can treat multiple sclerosis (MS) [12]
  • Can treat depression [12]
  • Can treat chronic pain [16]
  • Improves mental well-being [12]

Simply put: vitamin D is important for bone health, but it is not the “super vitamin” that many may claim it to be.

Vitamin C

Vitamin C is a highly popular nutritional supplement. It is frequently marketed as being important for promoting general health and disease prevention. While severe vitamin C deficiency is related to known diseases (e.g. scurvy), its usefulness as a supplement for healthy adults is questionable.

One of the most commonly-promoted uses for vitamin C is in the prevention and treatment of the common cold. Unfortunately, the best available evidence does not support this for the general population [17]. A systematic review of dozens of clinical trials demonstrated that regular supplementation with vitamin C has no effect on the chance that an average person will get a cold [17]. The same study also found that vitamin C was unable to reliably treat a cold after its onset (i.e. only taking vitamin C when you have a cold does not work as a treatment). However, regular vitamin C supplementation was found to slightly reduce the duration of colds, but the relevance of the effect was questioned by the study’s authors [17]. Essentially, an average adult would need to take vitamin C every day (i.e. 365 days a year) in order to achieve an 8% reduction in the duration of any colds they may get. This means that if a cold normally lasts 7 days, its duration would only be reduced by a fraction of a day. Whether or not consuming vitamin C supplements every day is worth less than a day’s reduction in cold symptoms may be a relevant consideration for consumers. Overall, however, the evidence does not suggest that vitamin C can prevent or treat the common cold [17].

There is also no convincing evidence that vitamin C supplementation can prevent or treat cancer [18-20], despite the fact that many natural/alternative health practitioners may claim otherwise [4]. Multiple clinical trials have demonstrated that a person’s chance of getting cancer is unaffected by vitamin C supplementation [18-20], while no studies have successfully proven an anti-tumor effect of vitamin C in humans.

Systematic reviews and clinical studies have also determined that:

  • Vitamin C supplementation does not prevent cardiovascular disease [21].
  • There is no conclusive evidence that vitamin C can prevent/treat pneumonia [22].
  • There is no evidence that vitamin C prevents age-related macular degeneration [23].
  • There is no reliable evidence that vitamin C can treat tetanus [24].
  • There is no reliable evidence that vitamin C can treat asthma [25-26].
  • There is no reliable evidence that vitamin C can prevent any other disease [6].

Despite the lack of evidence, alternative health practitioners advertise intravenous (IV) vitamin C injections as a therapy for many illnesses, including cancer [4]. The ethics of this practice are highly questionable.

Vitamins C and D



Multivitamin Claims

Vitamin D

Vitamin D Claims

Vitamin C

Vitamin C Claims


Vitamin supplementation may not be safe if doses are regularly taken above their “tolerable upper limit” (UL: the highest daily dose that is considered unlikely to cause adverse effects) [27-28]. Each vitamin has its own UL and associated toxicities. For example, high intakes of vitamin C are associated with kidney stone formation [29], while excess vitamin D may cause hypercalcemia (abnormally high calcium levels in the blood).

As mentioned above, regular supplementation with certain vitamins may be harmful. Clinical studies and systematic reviews have demonstrated that beta-carotene and vitamin E supplementation likely increase mortality (i.e. risk of death), and the same may be true of vitamin A [6, 11]. Medical practice guidelines therefore recommend against beta-carotene and vitamin E supplementation [8].


There is no good reason for average healthy adults to take (or purchase) routine vitamin supplements, unless directed to do so by a healthcare provider.

In healthy people, multivitamins do not promote good health or prevent disease.

When it comes to vitamins, more is not better. Some vitamins can cause harm in high doses.


1)  Guallar E, Stranges S, Mulrow C, Appel LJ, Miller ER. Enough is enough: Stop wasting money on vitamin and mineral supplements. Annals of Internal Medicine. 2013;159(12):850–851. doi:10.7326/0003-4819-159-12-201312170-00011.

2)  Fleming SA, Gutknecht NC. Naturopathy and the primary care practice. Primary Care: Clinics in Office Practice. 2010;37(1):119–136. doi:10.1016/j.pop.2009.09.002.

3)  Caulfield T, Rachul C. Supported by science? What canadian naturopaths advertise to the public. Allergy, Asthma & Clinical Immunology. 2011;7(1):14. doi:10.1186/1710-1492-7-14.

4)  Padayatty SJ, Sun AY, Chen Q, Espey MG, Drisko J, Levine M. Vitamin C: Intravenous use by complementary and alternative medicine practitioners and adverse effects. PLoS ONE. 2010;5(7):e11414. doi:10.1371/journal.pone.0011414.

5)  Fortmann SP, Burda BU, Senger CA, Lin JS, Whitlock EP. Vitamin and mineral supplements in the primary prevention of cardiovascular disease and cancer: An updated systematic evidence review for the U.S. Preventive services task force. Annals of Internal Medicine. 2013;159(12):824–834. doi:10.7326/0003-4819-159-12-201312170-00729.

6)  Bjelakovic G, Nikolova D, Gluud LL, Simonetti RG, Gluud C. Antioxidant supplements for prevention of mortality in healthy participants and patients with various diseases. Cochrane Database of Systematic Reviews. March 2012. doi:10.1002/14651858.cd007176.pub2.

7)  Huang H-Y et al. The efficacy and safety of Multivitamin and mineral supplement use to prevent cancer and chronic disease in adults: A systematic review for a national institutes of health state-of-the-science conference. Annals of Internal Medicine. 2006;145(5):372. doi:10.7326/0003-4819-145-5-200609050-00135.

8)  Moyer VA. Vitamin, mineral, and Multivitamin supplements for the primary prevention of cardiovascular disease and cancer: U.S. Preventive services task force recommendation statement. Annals of Internal Medicine. 2014;160(8):558. doi:10.7326/m14-0198.

9)  Grodstein F, O’Brien J, Kang JH, et al. Long-term Multivitamin Supplementation and cognitive function in men. Annals of Internal Medicine. 2013;159(12):806–814. doi:10.7326/0003-4819-159-12-201312170-00006.

10)  Miller ER, Juraschek S, Pastor-Barriuso R, Bazzano LA, Appel LJ, Guallar E. Meta-Analysis of Folic acid Supplementation trials on risk of cardiovascular disease and risk interaction with baseline Homocysteine levels. The American Journal of Cardiology. 2010;106(4):517–527. doi:10.1016/j.amjcard.2010.03.064.

11)  Miller ER et al. Meta-Analysis: High-dosage vitamin E supplementation may increase all-cause mortality. Annals of Internal Medicine. 2005;142(1):37. doi:10.7326/0003-4819-142-1-200501040-00110.

12)  Allan GM, Cranston L, Lindblad A, et al. Vitamin D: A narrative review examining the evidence for Ten beliefs. Journal of General Internal Medicine. 2016;31(7):780–791. doi:10.1007/s11606-016-3645-y.

13)  Health Canada: Vitamin D and calcium: Updated dietary reference Intakes - nutrition and healthy eating.

14)  National Academies of Sciences. Report Brief: Health and Medicine Division – Dietary reference intakes for calcium and vitamin D.

15)  Bjelakovic G, Gluud LL, Nikolova D, et al. Vitamin D supplementation for prevention of cancer in adults. Cochrane Database of Systematic Reviews. June 2014. doi:10.1002/14651858.cd007469.pub2.

16)  Straube S, Derry S, Straube C, Moore RA. Vitamin D for the treatment of chronic painful conditions in adults. Cochrane Database of Systematic Reviews. May 2015. doi:10.1002/14651858.cd007771.pub3.

17)  Hemilä H, Chalker E. Vitamin C for preventing and treating the common cold. Cochrane Database of Systematic Reviews. January 2013. doi:10.1002/14651858.cd000980.pub4.

18)  Gaziano JM, Glynn RJ, Christen WG, et al. Vitamins E and C in the prevention of prostate and total cancer in men. JAMA. 2009;301(1):52. doi:10.1001/jama.2008.862.

19)  Lin J, Cook NR, Albert C, et al. Vitamins C and E and beta Carotene Supplementation and cancer risk: A Randomized controlled trial. JNCI Journal of the National Cancer Institute. 2009;101(1):14–23. doi:10.1093/jnci/djn438.

20)  Wang L, Sesso HD, Glynn RJ, et al. Vitamin E and C supplementation and risk of cancer in men: Posttrial follow-up in the physicians’ health study II randomized trial. The American Journal of Clinical Nutrition. 2014;100(3):915–923. doi:10.3945/ajcn.114.085480.

21)  Sesso HD et al. Vitamins E and C in the prevention of cardiovascular disease in men. JAMA. 2008;300(18):2123. doi:10.1001/jama.2008.600.

22)  Hemilä H, Louhiala P. Vitamin C for preventing and treating pneumonia. Cochrane Database of Systematic Reviews. August 2013. doi:10.1002/14651858.cd005532.pub3.

23)  Evans JR, Lawrenson JG. Antioxidant vitamin and mineral supplements for preventing age-related macular degeneration. Cochrane Database of Systematic Reviews. June 2012. doi:10.1002/14651858.cd000253.pub3.

24)  Hemilä H, Koivula T. Vitamin C for preventing and treating tetanus. Cochrane Database of Systematic Reviews. November 2013. doi:10.1002/14651858.cd006665.pub3.

25)  Milan SJ, Hart A, Wilkinson M. Vitamin C for asthma and exercise-induced bronchoconstriction. Cochrane Database of Systematic Reviews. October 2013. doi:10.1002/14651858.cd010391.pub2.

26)  Milan SJ, Wilkinson M, Hart A, Sugumar K. Vitamins C and E for asthma and exercise-induced bronchoconstriction. Cochrane Database of Systematic Reviews. June 2014. doi:10.1002/14651858.cd010749.pub2.

27)  National Institutes of Health (NIH) Office of Dietary Supplements. Nutrient Recommendations: Dietary Reference Intakes (DRI).

28)  Health Canada. Reference values for vitamins - dietary reference intakes (DRI) tables.

29)  Thomas LDK, Elinder C-G, Tiselius H-G, Wolk A, Åkesson A. Ascorbic acid supplements and kidney stone incidence among men: A prospective study. JAMA Internal Medicine. 2013;173(5):386. doi:10.1001/jamainternmed.2013.2296.